Method of operating reactor

ABSTRACT

A method of operating a nuclear reactor reduces the number of reload fuels to be loaded into the nuclear reactor in the second and the following operation cycles. 
     The nuclear reactor has a reactor core in which a plurality of reload fuel assemblies respectively having different infinite multiplication factors are arranged. The method operates the nuclear reactor with control rods inserted in control cells each comprising four reload fuel assemblies having relatively large infinite multiplication factors among the plurality of reload fuel assemblies for a period longer than half of a period of an operation cycle.

TECHNICAL FIELD

The present invention relates to a method of operating a boiling-waterreactor which is and, more particularly, to a method of operating aboiling-water reactor suitable for the second and the following cycles.

BACKGROUND OF THE INVENTION

Generally, some initial loading fuel assemblies (hereinafter referred toas “initial loading fuels”) are taken out of the reactor core of anuclear reactor after the completion of an operation for the first cycleand are replaced with new reload fuel assemblies (hereinafter referredto as “reload fuels”). The fuel assemblies taken out of the nuclearreactor have burn-ups which are smaller than those of other fuelassemblies and so they generate a small amount of energy.

A method of operating a nuclear reactor, as disclosed in Japanese PatentLaid-open No. Hei 3-214097, loads at least fuel assemblies taken out ofthe reactor core of the nuclear reactor after the completion of thesecond cycle again into the reactor core when exchanging the fuel afterthe completion of the third cycle to increase the discharge exposure ofthe initial loading fuels and to reduce the number of reload fuels.

A technique, as disclosed in Japanese Patent Laid-open No. Hei 4-249794,controls the operation of a nuclear reactor by using only control rodsadjacent to fuel assemblies not having a minimum enrichment factor andincluded in initial loading fuels loaded into the initial reactor coreby arranging fuel assemblies having large enrichment factors in aperipheral part of the initial reactor core and by arranging fuelassemblies having small enrichment factors in a central part of theinitial reactor core to increase the burn-up of the fuel assembliesremoved from the reactor core after the completion of the first cycle.

A prior art technique, as disclosed in Japanese Patent Laid-open No. Hei6-186372, constructs each of the control cells by using four fuelassemblies, including fuel assemblies having large infinitemultiplication factors (hereinafter referred to as“large-infinite-multiplication fuels”), inserts control rods into thecontrol cells in response to a decrease of the infinite multiplicationfactor of the large-infinite-multiplication-factor fuels below the meaninfinite multiplication factor in the reactor core in one cycle, andoperates the nuclear reactor in this state for the remaining period ofthe cycle.

It is mentioned in Japanese Patent Laid-Open No. Hei 6-186372 that manyfuel assemblies having small infinite multiplication factors(hereinafter referred to as “small-infinite-multiplication-factorfuels”) are arranged outside the control cell, and so a sufficientlylarge number of the small-infinite-multiplication-factor fuel assembliescan be arranged in the outer peripheral region of the reactor core, andhence the reactor core permits only slight neutron leakage. It is alsomentioned in this cited reference that the construction of the controlcells by assembling the fuel assemblies includingsmall-infinite-multiplication factor fuels and the arrangement of asufficiently large number of small-infinite-multiplication-factor fuelsin the outer peripheral region of the reactor core, as compared with theconstruction of control cells by assembling onlysmall-infinite-multiplication-factor fuels and the arrangement of aninsufficient number of small-infinite-multiplication-factor fuels in theperipheral region of the reactor core, are effective in improving fueleconomy.

The prior art technique mentioned in Japanese Patent Laid-open No. Hei3-214097 loads the initial loading fuels again into the reactor core toincrease the burn-up of the initial loading fuels and considers nomeasures for increasing the burn-up of reload fuels.

The prior art technique mentioned in Japanese Patent Laid-open No. Hei4-249794 gives no consideration to the second and the following cycles.

The prior art technique mentioned in Japanese Patent Laid-open No. Hei6-186372 gives no consideration to the reduction of the number of reloadfuels.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method ofoperating a nuclear reactor, which method is capable of reducing thenumber of reload fuels to be loaded into the nuclear reactor in thesecond and the following cycles.

With the foregoing object in view, according to the present invention, amethod of operating a nuclear reactor having a reactor core, in which aplurality of reload fuel assemblies respectively having differentinfinite multiplication factors are arranged, inserts control rods incontrol cells each comprising four reload fuel assemblies havingrelatively large infinite multiplication factors among the plurality ofreload fuel assemblies for a period longer than half of the period of anoperation cycle.

According to the present invention, a method of operating a nuclearreactor having a reactor core, in which a plurality of reload fuelassemblies respectively having different infinite multiplication factorsand initial loading fuel assemblies are arranged, inserts control rodsin control cells each comprising four reload fuel assemblies havingrelatively large infinite multiplication factors among the plurality ofreload fuel assemblies and the initial loading fuel assemblies in aperiod longer than half of the period of an operation cycle.

The burn-up of the fuel assemblies forming the control cells into whichthe control rods are inserted is suppressed by the control action of thecontrol rods. Accordingly, when the nuclear reactor is operated with thecontrol rods inserted into the control cells each comprisinglarge-infinite-multiplication-factor fuels, the fuel assemblies of thecontrol cells have large infinite multiplication factors in the nextcycle. Since the fuel assemblies having large infinite multiplicationfactors can be used in the next cycle, the number of the reload fuelscan be reduced by a number equal to the number of those fuel assemblieshaving large infinite multiplication factors.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a quarter sectional view of a fuel loading pattern for anequilibrium cycle formed by a method representing a first embodimentaccording to the present invention;

FIG. 2 is a quarter sectional view of a fuel loading pattern for anequilibrium cycle formed by a method representing a comparative examplecorresponding to the first embodiment;

FIGS. 3(a) to 3(c) patterns in which control rods are arranged for anequilibrium cycle by the method in the first embodiment;

FIG. 4 is a quarter sectional view of a fuel loading pattern for a thirdcycle formed by a method representing a second embodiment according tothe present invention; and

FIGS. 5(a) to 5(c) are diagrams of patterns in which control rods arearranged for the third cycle by the method in the second embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described withreference to the accompanying drawings.

First, a state where only reload fuels are placed in the reactor core ofa nuclear reactor will be explained.

FIG. 1 is a quarter sectional view of a fuel loading pattern for anequilibrium cycle formed by a method representing a first embodimentaccording to the present invention.

Referring to FIG. 1, an equilibrium core comprises 872 reload fuelassemblies including 196 fuel assemblies for the first cycle(hereinafter referred to as “first cycle fuels”), 196 fuel assembliesfor the second cycle (hereinafter referred to as “second cycle fuels”),196 fuel assemblies for the third cycle (hereinafter referred to as“third cycle fuels”), 196 fuel assemblies for the fourth cycle(hereinafter referred to as “fourth cycle fuels”n), and 88 fuelassemblies for the fifth cycle (hereinafter referred to a “fifth cyclefuels”). The mean enrichment factor of the reload fuel assemblies isabout 3.8 wt %.

The reactor core is provided with twenty-four first control cells eachconsisting of four little fissioned fuel assemblies having a relativelylarge infinite multiplication factor, and thirteen second control cells20 each consisting of four considerably fissioned fuel assemblies havinga relatively small infinite multiplication factor.

The first control cell has first cycle fuels 1, second cycle fuels 2 andthird cycle fuels 3. The second control cell 20 has fourth cycle fuels 4and fifth cycle fuels 5.

The first control cells are divided into two groups differing from eachother in the operation of the control rods; twelve first control cells11 of a group A (hereinafter referred to a “first control cells (A)”)and twelve first control cells 12 of a group B (hereinafter referred toas “first control cells (B)”).

The first cycle fuels 1, the second cycle fuels 2 and the third cyclefuels 3 are disposed in regions other than an outer peripheral region ofthe reactor core and regions for the second control cells 20. The fifthcycle fuels 5 are disposed in the outer peripheral region and in theregions for the second control cells 20. The fourth cycle fuels 4 aredisposed in the outer peripheral region of the reactor core, the regionsfor the second control cells 20, regions providing relatively highoutput and outer regions of the reactor core.

After an operation cycle has been completed, the first cycle fuels 1 aremoved to positions from which the second cycle fuels 2 have beenremoved, the second cycle fuels 2 are moved to positions from which thethird cycle fuels have been removed, the third cycle fuels 3 are movedto positions from which the fourth cycle fuels 4 have been removed andnew reload fuels are disposed at positions from which the first cyclefuels 1 have been removed.

After the completion of the operation cycle, the 116 considerablyfissioned fourth cycle fuels 4 among the fourth cycle fuels 4 and allthe fifth cycle fuels 5 are removed from the reactor core, and theremaining 80 fourth cycle fuels 4 are moved to positions from which thefifth cycle fuels 5 have been removed.

An operation is continued for most of the period of an operation cyclewith cruciform control rods, not shown, inserted in the first controlcells (A) 11 and (B) 12 and without inserting any control rods in thesecond control cells 20; that is, the control rods are inserted in onlythe first control cells (A) 11 and (B) 12 for most of the period of theoperation cycle and the nuclear reactor is operated.

A control rod pattern, i.e., a pattern indicating the control rodsinserted in the control cells, in the first embodiment will be describedwith reference to FIGS. 3(a), 3(b) and 3(c) are quarter sectional viewsof control rod patterns in which, which control rods are arranged by themethod in the first embodiment. In FIGS. 3(a), 3(b) and 3(c), thecontrol rods are inserted in the shaded control cells.

In an initial stage of the cycle, in which burn-up is in the range of 0to 2.2 GWd/t, the nuclear reactor is operated with the control rodsinserted in the thirteen second control cells 20 as shown in FIG. 3(a)In most of the remaining period of the cycle, in which burn-up is in therange of 3.3 to 9.4 GWd/t, the nuclear reactor is operated with thecontrol rods inserted in only the twenty four first control cells (A) 11and (B) 12 as shown in FIG. 3(b). In the last stage of operation of thenuclear reactor, in which burn-up is 10.4 GWd/t, all the control rodsare extracted from the reactor core as shown in FIG. 3(c).

In most of the period of the cycle, the nuclear reactor is operated withthe control rods inserted alternately in the first control cells (A) 11and the first control cells (B) 12. First, the control rods are insertedin only the first control cells (A) 11 for a period in which the nuclearreactor operates at a burn-up of several gigawatts day per ton, and thenthe control rods are extracted from all the first control cells (A) 11and control rods are inserted in only the first control cells (B) 12 fora period in which the nuclear reactor operates at several gigawatts dayper ton. Thereafter, this operating mode is repeated.

Generally, asymmetric burn-up occurs in the fuels of the fuel assemblieswhen the control rod is inserted in the control cell. Therefore,when thecontrol rod is extracted, then output increases locally in the fuel rodsadjacent to the control rod, causing control blade historical effectwhich adversely affects the soundness of the fuel rods.

This embodiment limits burn-up in a period in which the control rods areinserted continuously in the first control cells (A) 11 and the firstcontrol cells (B) 12 to several gigawatts day per ton to reduce thecontrol blade historical effect which occurs when the control rods areextracted.

A method in a comparative example for verifying the effect of the firstembodiment operates the nuclear reactor through a cycle with the controlrods inserted in only the second control cells 20 each consisting of thefour fuel assemblies having relatively small infinite multiplicationfactors.

An equilibrium core for carrying out the comparative example comprises872 reload fuel assemblies including 200 first cycle fuels 1, 200 secondcycle fuels 2, 200 third cycle fuels 3, 200 fourth cycle fuels 4, and 72fifth cycle fuels 5.

The mean enrichment factor of the reload fuels, the numbers and thearrangement of the first control cells (A) 11 and (B) 12, and the numberand the arrangement of the second control cells 20 are the same as thosein the reactor core for carrying out the first embodiment.

In this reactor core, the first cycle fuels 1, the second cycle fuels 2and the third cycle fuels 3 are disposed in regions other than an outerperipheral region of the reactor core and regions for the second controlcells 20. The fifth cycle fuels 5 are disposed in the outer peripheralregion and in the regions for the second control cells 20. The fourthcycle fuels 4 are disposed in the outer peripheral region of the reactorcore, the regions for the second control cells 20, regions providingrelatively high output and outer regions of the reactor core.

After an operation cycle has been completed, the first cycle fuels 1 aremoved to positions from which the second cycle fuels 2 have beenremoved, the second cycle fuels 2 are moved to positions from which thethird cycle fuels have been removed, and the third cycle fuels are movedto positions from which the fourth cycle fuels 4 have been removed. Newreload fuels are disposed at positions from which the first cycle fuelshave been removed.

After the operation cycle has been completed, 128 considerably fissionedfourth cycle fuels 4 having a relatively small infinite multiplicationfactor among the fourth cycle fuels 4 are removed from the reactor core,and the remaining 72 fourth cycle fuels 4 are moved to positions fromwhich the fifth cycle fuels 5 have been removed. All the fifth cyclefuels 5 are removed from the reactor core.

In this comparative example, the control rods are inserted in the secondcontrol cells 20 throughout the cycle of operation of the nuclearreactor (FIGS. 3(a) and 3(b)), and all the control rods are extractedfrom the reactor core in the last stage of the cycle (FIG. 3(c)).

When the nuclear reactor is operated by the method in the firstembodiment, the number of new reload fuels to be loaded into the reactorcore after the completion of the cycle is smaller by four than that ofthe new reload fuels to be loaded into the reactor core after thecompletion of the cycle when the nuclear reactor is operated by themethod in the comparative example.

Such an effect can be provided by operating the nuclear reactor with thecontrol rods inserted in the first control cells for a period (FIG.3(b)) longer than half of the period of the cycle (FIGS. 3(a) to 3(c));that is, the effect can be provided even if the control rods areinserted in the first control cells throughout the cycle (FIGS. 3(a) and3(b)).

A fuel arrangement including both the initial loading fuels and thereload fuels for the third cycle will be explained by way of example.

FIG. 4 is a quarter sectional view of a fuel loading pattern for thethird cycle formed by a method representing a second embodimentaccording to the present invention. The reactor core of a nuclearreactor is provided with 208 low enrichment fuel assemblies (hereinafterreferred to as “low-enrichment fuels”) having a mean enrichment factorof about 1.5 wt % at initial loading and 664 high-enrichment fuelassemblies (hereinafter referrer to as “high enrichment fuels”) having amean enrichment factor of about 4.1 wt % at initial loading, and noreload fuels are loaded into the reactor core in the second cycle.

The reactor core in the third cycle as shown in FIG. 4 comprises 872fuel assemblies including 132 first cycle fuels 1, 76 low-enrichmentfuels la (initial loading fuels) and 664 high-enrichment fuels 1 b(initial loading fuels). The first cycle fuels 1, i.e., reload fuels,are fuel assemblies having a mean enrichment factor of about 3.8 wt %.

Forty first control cells each comprising the first cycle fuels 1 andthe fissioned high-enrichment fuels having relatively large infinitemultiplication factors, and thirty-seven second control cells 40 eachcomprising considerably fissioned high-enrichment fuels havingrelatively small infinite multiplication factors are formed in thereactor core.

The first control cells are divided into two groups differing from eachother in the operation of the control rods; sixteen first control cells(A) 31 and twenty-four first control cells (B) 32.

The first cycle fuels 1 are disposed in a central region of the reactorcore, the low-enrichment fuels 1 a are arranged in an outer peripheralregion of the reactor core, and the high-enrichment fuels 1 b aredisposed in the rest remaining regions in the reactor core.

After the completion of this operation cycle, i.e., the third cycle, the216 fuels including all the 76 low enrichment fuels 1 a, and the 140considerably fissioned high-enrichment fuels 1 b having small infinitemultiplication factors among the high-enrichment fuels 1 b are removedfrom the reactor core and 216 new reload fuels are loaded into thereactor core.

In most of the period of the third cycle, the nuclear reactor isoperated with cruciform control rods, not shown, inserted in the firstcontrol cells (A) 31 and (B) 32, and without inserting any control rodsin the second control cells 40. That is, the nuclear reactor is operatedfor the most part of the period of the third cycle with the control rodsinserted in only the first control cells (A) 31 and (B) 32.

A control rod pattern in the second embodiment will be described withreference to FIGS. 5(a), 5(b) and 5(c), which are quarter sectionalviews of control rod patterns in which control rods are arranged by themethod in the second embodiment. In FIGS. 5(a). 5(b) and 5(c), thecontrol rods are inserted in the shaded control cells.

In an initial stage of the cycle, in which burn-up is in the range of 0to 2.2 GWd/t, the nuclear reactor is operated with the control rodsinserted in the twenty second control cells 40 as shown in FIG. 5(a). Inmost of the remaining period of the cycle, in which burn-up is in therange of 3.3 to 9.4 GWd/t, the nuclear reactor is operated with thecontrol rods inserted in only the forty first control cells (A) 31 and(B) 32 as shown in FIG. 5(b). In the last stage of the cycle, in whichburn-up is 10.4 GWd/t, all the control rods are extracted from thereactor core as shown in FIG. 5(c).

In most of the period of the cycle, the method in the second embodiment,similarly to the method in the first embodiment, operates the nuclearreactor by inserting the control rods alternately in the first controlcells (A) 31 and the first control cells (B) 32 for a period in whichburn-up is several gigawatts day per ton. Thus, the second embodiment,similarly to the first embodiment, reduces control blade historicaleffect that occurs when the control rods are extracted.

A method in a comparative example for verifying the effect of the secondembodiment operates the nuclear reactor through a cycle with the controlrods inserted in only the second control cells 40 each consisting of thefour fuel assemblies having the same loading patterns and relativelysmall infinite multiplication factors. The mode of operation in thefirst and the second cycle and the movement of the fuel assemblies afterthe completion of the second cycle by the method in the comparativeexample is the same as those by the method in the second embodiment.

The method in the comparative example operates the nuclear reactor byinserting the control rods only in the second control cells 40throughout the third cycle (FIGS. 5(a) and 5(b)), and extracts all thecontrol rods from the reactor core in the last stage of the cycle (FIG.5(c)).

The method in the comparative example removes 220 fuels including allthe low-enrichment fuels 1 a and 144 considerably fissionedhigh-enrichment fuels 1 b having small infinite multiplication factorsamong the high enrichment fuels 1 b after the completion of the thirdcycle and loads 220 new reload fuels into the reactor core.

Accordingly, when the nuclear reactor is operated by the method in thesecond embodiment, the number of the new reload fuels to be loaded intothe reactor core after the completion of the cycle is smaller by fourthan that of the new reload fuels to be loaded into the reactor coreafter the completion of the cycle when the nuclear reactor is operatedby the method in the comparative example.

Such an effect can be provided by operating the nuclear reactor with thecontrol rods inserted in the first control cells for a period (FIG.5(b)) longer than half of the period of the cycle (FIGS. 5(a) to 5(c));that is, the effect can be provided even if the control rods areinserted in the first control cells throughout the cycle (FIGS. 5(a) and5(b)).

The difference in the number of the reload fuels between the firstembodiment and the second embodiment, and the comparative examples, andthe criticality of the nuclear reactor will be explained below.

As mentioned above, the burn-up of the fuel assemblies forming thecontrol cells into which the control rods are inserted is suppressed bythe control action of the control rods.

The method in the foregoing embodiment operates the nuclear reactor withthe control rods inserted in the control cells each comprising thelarge-infinite-multiplication-factor fuels for most of the period of oneoperation cycle. Accordingly, the fuel assemblies of the first controlcells still have enough infinite multiplication factors after thecompletion of the operation cycle.

Since the fuel assemblies having large infinite multiplication factorscan be used in the next cycle, the nuclear reactor can be kept in acritical state in the next cycle even if the number of the new reloadfuels is smaller than that of the new reload fuels needed by the methodin the comparative example.

In the foregoing embodiment, most or all of the fuel assemblies takenout of the reactor core after the completion of the cycle are thosedisposed in regions other than those in which the control cells aredisposed. Accordingly, the burn-up of the fuel assemblies to be takenout of the nuclear reactor is promoted and the burn-up can be enhanced.

What is claimed is:
 1. A method of operating a nuclear reactor having areactor core in which a plurality of reload fuel assemblies respectivelyhaving different infinite multiplication factors are arranged, saidreload fuel assemblies including small-number cycle reload fuelassemblies having a larger infinite multiplication factor than alarge-number operation cycle reload fuel assemblies having a smallerinfinite multiplication factor than the larger infinite multiplicationfactor of said small-number cycle reload fuel assemblies, thesmall-number cycle reload fuel assemblies having been operated for asmaller number of operation cycles than the number of cycles ofoperation of said large-number cycle reload fuel assemblies, said methodcomprising: inserting control rods alternately in control cells of afirst group and of a second group with respect to time, each controlcell of the first group and the second group comprising foursmall-number cycle reload fuel assemblies having the larger infinitemultiplication factor for a period longer than half of a period of anoperation cycle.
 2. A method of operating a nuclear reactor having areactor core in which a plurality of reload fuel assemblies respectivelyhaving different infinite multiplication factors and a plurality ofinitial loading fuel assemblies comprising high-enrichment initialloading fuel assemblies and low-enrichment initial loading fuelassemblies with a lower enrichment factor than an enrichment factor ofthe high-enrichment initial loading fuel assemblies are arranged, saidmethod comprising: inserting control rods alternately in control cellsof a first group and of a second group with respect to time, eachcontrol cell of the first group and the second group comprising fourfuel assemblies including at least one reload fuel assembly and aplurality of high-enrichment initial loading fuel assemblies for aperiod longer than half of a period of an operation cycle.
 3. The methodof operating a nuclear reactor according to claim 1 or 2, wherein thecontrol cells in which the control rods are inserted include new fuelassemblies loaded into the reactor core for the operation cycle.
 4. Themethod of operating a nuclear reactor according to claim 1 or 2, whereinthe control rods are inserted in the control cells of each group at timeintervals corresponding to a burn-up of several gigawatts day per ton.5. The method of operating a nuclear reactor according to claim 4,wherein the fuel assemblies other than those forming the control cellsin which the control rods are inserted are removed from the reactor coreafter the completion of the operation cycle.
 6. The method of operatinga nuclear reactor according to claim 4, wherein most of the fuelassemblies arranged in an outer peripheral region of the reactor coreare those having small infinite multiplication factors.
 7. The method ofoperating a nuclear reactor according to claim 1, wherein the fuelassemblies other than those forming the control cells in which thecontrol rods are inserted are removed from the reactor core after thecompletion of the operation cycle.
 8. The method of operating a nuclearrector according to claim 2, wherein the fuel assemblies other thanthose forming the control cells in which the control rods are insertedare removed from the reactor core after the completion of the operationcycle.
 9. The method of operating a nuclear reactor according to claim3, wherein the fuel assemblies other than those forming the controlcells in which the control rods are inserted are removed from thereactor core after the completion of the operation cycle.
 10. The methodof operating a nuclear reactor according to claim 1, wherein most of thefuel assemblies arranged in an outer peripheral region of the reactorcore are those having small infinite multiplication factors.
 11. Themethod of operating a nuclear reactor according to claim 2, wherein mostof the fuel assemblies arranged in an outer peripheral region of thereactor core are those having small infinite multiplication factors. 12.The method of operating a nuclear reactor according to claim 3, whereinmost of the fuel assemblies arranged in an outer peripheral region ofthe reactor core are those having small infinite multiplication factors.13. A method of operating a nuclear reactor having a reactor core inwhich a plurality of reload fuel assemblies comprising small-numbercycle reload fuel assemblies and large-number cycle reload fuelassemblies having been operated for a larger number of operation cyclesthan a number of operation cycles of the small-number cycle reload fuelassemblies are arranged, said method comprising: inserting control rodsalternately in control cells of a first group and of a second group withrespect to time, each control cell of the first group and the secondgroup comprising four reload fuel assemblies of the small-number cyclereload fuel assemblies for a period longer than half of a period of anoperation cycle.
 14. A method of operating a nuclear reactor having areactor core in which a plurality of reload fuel assemblies and aplurality of initial loading fuel assemblies comprising high-enrichmentinitial loading fuel assemblies and low-enrichment initial loading fuelassemblies with a lower enrichment factor than an enrichment factor ofthe high-enrichment initial loading fuel assemblies are arranged, saidmethod comprising: inserting control rods alternately in control cellsof a first group and those of a second group with respect to time, eachcontrol cell of the first group and the second group comprising fourfuel assemblies formed of at least one of the reload fuel assemblies anda plurality of the high-enrichment initial loading fuel assemblies for aperiod longer than half of a period of an operation cycle.
 15. Themethod of operating a nuclear reactor according to claim 1 or 13,wherein the number of operation cycles of the small-number cycle reloadfuel assemblies is not greater than three and the number of operationcycles of the large-number cycle reload fuel assemblies is greater thanthree.
 16. The method of operating a nuclear reactor according to claim2 or 14, wherein each control cell of the first group and the secondgroup comprise no more than two of the reload fuel assemblies.